In the oil and gas exploration and production industry, a wellbore or borehole of an oil or gas well is typically drilled from surface to a first depth and lined with a steel casing which is cemented in place. The borehole is then extended and a further section of smaller diameter casing is located in the extended section and also cemented in place. This process is repeated until the wellbore has been extended to a desired depth, intersecting a producing formation. In an alternative, tubing known as a liner is located in the borehole, extending from the deepest casing section to a producing formation, and is also cemented in place. The well is then completed by locating a string of production tubing within the casing/liner, through which well fluids flow to surface.
Before the well can be completed, it is necessary to clean the lined wellbore and to replace the fluids present in the wellbore with a completion fluid such as brine. The cleaning process serves to remove solids adhered to the wall of the casing or liner; to circulate residual drilling mud and other fluids out of the wellbore; and to filter out solids present in the wellbore fluid. A considerable amount of debris in the wellbore and on the surface of the casing/liner comprises rust particles and metal chips or scrapings originating from equipment used in the well and the casing or liner itself.
A cleaning operation typically involves carrying out a mechanical cleaning procedure, where an abrasive cleaning tool is reciprocated back and forth within the wellbore tubing, to remove the solids adhered to the tubing wall. Other cleaning procedures may involve jetting fluid on to a wall of the wellbore tubing at a desired location using a circulation tool, to assist in solid particle removal, and to circulate the solids to surface. Typically, a tool string is assembled which incorporates one or more mechanical cleaning tools and a circulation tool. Following a mechanical cleaning of the wellbore tubing, the circulation tool is activated at a desired location, to jet fluid on to the wellbore tubing wall to further clean the tubing.
In order to achieve this, it is necessary to provide a circulation tool which can be selectively activated downhole. One such suitable circulation tool is disclosed in the Applicant's International Patent Application No. PCT/GB2004/001449, published as WO 2004/088091. The circulation tool disclosed in WO 2004/088091 is activated to circulate fluid from an internal bore of the tool to the tool exterior by dropping valve members in the form of balls into the tool. The balls seat on a ball seat of the tool, to selectively close fluid flow through a main bore of the tool, thereby permitting movement of an internal sleeve to open flow to the tool exterior. The tool can be repeatedly cycled to open and close flow to the tool exterior by dropping a succession of balls, which are blown through the ball seat to permit further operations. This is typically achieved by providing a deformable ball seat, although deformable balls may be utilised.
Whilst the circulation tool disclosed in WO 2004/088091 is effective at circulating fluid to the tool exterior, it is desired to improve upon the operation of the tool and the methods utilised for circulating fluid disclosed therein. In particular, the deformable materials utilised in the manufacture of the deformable ball seat/balls can be affected by changing downhole conditions, such as variations in temperature and pressure. This can lead to variations in the operating parameters of the tool.
It is also desired to improve upon other features of the tool disclosed in WO 2004/088091. For example, the circulation tool of WO 2004/088091 requires that an index sleeve be cycled back and forth within a main bore of the tool, to permit repeated opening and closing of fluid flow to the tool exterior when balls are seated on the ball seat. The sleeve is biased by a spring located in a spring chamber defined between an outer body of the tool and the indexing sleeve. This chamber must be open to fluid ingress/egress, in order to permit pressure equalisation during running-in and pulling-out of the tool. Over time, repeated cycling of the indexing sleeve results in the ingress of solids-laden fluids, particularly drilling fluids. The solids in these fluids have been found to settle out over time, and can restrict movement of the indexing sleeve and/or operation of the biasing spring.
Additionally, the circulation tool of WO 2004/088091 includes indexing pins or dogs which govern the axial and rotational position of the indexing sleeve relative to the tool outer body. These dogs are of a conventional type, and are cylindrical in shape. Whilst cylindrical dogs of this type are effective in cycling the indexing sleeve, it has been found that the circular section of the pins does not provide the optimum force transfer to the indexing sleeve, and increases the chance of dog fracture over time.
It is therefore amongst the objects of embodiments of the present invention to obviate or mitigate at least one of the foregoing disadvantages.